Exhaust aftertreatment systems receive and treat exhaust gas generated by an internal combustion engine. Typical exhaust aftertreatment systems include any of various components configured to reduce the level of harmful exhaust emissions present in the exhaust gas. For example, some exhaust aftertreatment systems for diesel powered internal combustion engines include various components, such as a diesel oxidation catalyst (DOC), particulate matter filter or diesel particulate filter (DPF), and a selective catalytic reduction (SCR) catalyst. In some exhaust aftertreatment systems, exhaust gas first passes through the diesel oxidation catalyst, then passes through the diesel particulate filter, and subsequently passes through the SCR catalyst.
Each of the DOC, DPF, and SCR catalyst components is configured to perform a particular exhaust emissions treatment operation on the exhaust gas passing through the components. Generally, the DOC reduces the amount of carbon monoxide and hydrocarbons present in the exhaust gas via oxidation techniques. The DPF filters harmful diesel particulate matter and soot present in the exhaust gas. Finally, the SCR catalyst reduces the amount of nitrogen oxides (NOx) present in the exhaust gas.
The SCR catalyst is configured to reduce NOx into less harmful emissions, such as N2 and H2O, in the presence of ammonia (NH3). Because ammonia is not a natural byproduct of the combustion process, it must be artificially introduced into the exhaust gas prior to the exhaust gas entering the SCR catalyst. Typically, ammonia is not directly injected into the exhaust gas due to safety considerations associated with the storage of gaseous ammonia. Accordingly, conventional systems are designed to inject a diesel exhaust fluid (DEF) or reductant into the exhaust gas, which is capable of decomposing into gaseous ammonia in the presence of exhaust gas under certain conditions. The DEF commonly used by conventional exhaust aftertreatment systems is a urea-water solution.
Generally, the decomposition of DEF into gaseous ammonia occupies three stages. First, DEF mixes with exhaust gas and water is removed from the DEF through a vaporization process. Second, the temperature of the exhaust causes a thermolysis-induced phase change in the DEF and decomposition of the DEF into isocyanic acid (HNCO) and NH3. Third, the isocyanic acid reacts with water in a hydrolysis process under specific pressure and temperature concentrations to decompose into ammonia and carbon dioxide (CO2). The gaseous ammonia is then introduced at the inlet face of the SCR catalyst, flows through the catalyst, and is consumed in the NOx reduction process. Any unconsumed ammonia exiting the SCR system can be reduced to N2 and other less harmful or less noxious components using an ammonia oxidation catalyst.
SCR systems typically include a DEF source and a DEF injector or doser coupled to the source and positioned upstream of the SCR catalyst. The DEF injector injects DEF into a decomposition space or tube through which an exhaust gas stream flows. Upon injection into the exhaust gas stream, the injected DEF spray is heated by the exhaust gas stream to trigger the decomposition of DEF into ammonia. As the DEF and exhaust gas mixture flows through the decomposition tube, the DEF further mixes with the exhaust gas before entering the SCR catalyst. Ideally, DEF is sufficiently decomposed and mixed with the exhaust gas prior to entering the SCR catalyst to provide an adequately uniform distribution of ammonia at the inlet face of the SCR catalyst.
Some prior art exhaust aftertreatment systems, however, do not provide adequate decomposition and mixing of injected DEF. Often, conventional systems cause exhaust gas recirculation within the DEF decomposition tube or low temperature regions within the decomposition tube. Exhaust gas recirculation and low temperature regions may result in inadequate mixing or decomposition, which may lead to the formation of solid DEF deposits on the inner walls of the decomposition tube and DEF injector. Solid DEF deposits include the solid byproducts from incomplete decomposition of urea, such as biuret, cyanuric acid, ammelide, and ammeline. Additionally, inadequate mixing may result in a low ammonia vapor uniformity index, which can lead to uneven distribution of the ammonia across the SCR catalyst surface, lower NOx conversion efficiency, and other shortcomings.
The formation of solid DEF deposits and uneven ammonia distribution may also be caused by DEF spray being deflected away from an intended target. Following injection, the DEF spray typically rapidly decelerates due to entrainment of exhaust gas into the spray. Rapid deceleration reduces DEF spray penetration and momentum, which makes the injected DEF spray susceptible to substantial redirection when contacted by exhaust flow gases. Undesirable redirection of DEF spray may result in DEF spray unintentionally contacting certain surfaces of the decomposition tube (e.g., an inner wall of the decomposition tube and an upper portion of a mixer) and forming solid DEF deposits thereon. The formation of solid DEF deposits within the decomposition tube typically results in a lower amount of ammonia concentration and a lower ammonia distribution uniformity index at the inlet face of the SCR catalyst, which can degrade the performance and control of the SCR catalyst. Additionally, solid DEF deposits in the decomposition tube can induce exhaust backpressure within the exhaust aftertreatment system, which can adversely impact the performance of the engine and exhaust aftertreatment system.
Solid DEF deposits within the decomposition tube can also indirectly trigger diagnostic faults associated with diagnostic signals that may be influenced by DEF deposits. For example, DEF deposits may cause a low NOx conversion rate, which may result in false decisions made by a NOx conversion efficiency monitor of an on-board diagnostics (OBD) system. Further, in exhaust aftertreatment systems employing a DPF, DEF deposits may act to increase the exhaust pressure at the outlet of the DPF, which may trigger a DPF outlet high pressure diagnostic fault. In this situation, the DPF outlet high pressure diagnostic fault is false because the DPF outlet high pressure condition is not a result of the condition of the DPF, but rather DEF deposit buildup downstream of the DPF.
Some conventional systems recognize the negative effect of DEF deposit buildup on the SCR catalyst. Such systems employ regeneration techniques to regenerate the SCR catalyst and remove the DEF deposits from the SCR catalyst. These systems, however, do not account for DEF deposit build-up within the DEF decomposition tube upstream of the SCR catalyst and the effect such DEF deposit build-ups can have on the OBD system.